Burimamide (IIa) was the first clinically effective histamine H.sub.2 -receptor antagonist. It inhibits gastric secretion in animals, including man, but oral absorption is poor. ##STR5## Metiamide (IIb), a subsequently evaluated histamine H.sub.2 -antagonist, is more potent than burimamide and is orally active in man. Clinical utility was limited, however, owing to toxicity (agranulocytosis). Cimetidine (IIc) is as effective an histamine H.sub.2 -antagonist as metiamide, without producing agranulocytosis, and has recently been marketed as an anti-ulcer drug.
Reviews on the development of histamine H.sub.2 -antagonists, including those discussed in the preceding paragraph, may be found in C. R. Ganellin, et al., Federation Proceedings, 35, 1924 (1976), in Drugs of the Future, 1, 13 (1976) and in references cited therein.
U.S. application Ser. No. 473,791, filed Mar. 16, 1983 discloses 3,4-disubstituted-1,2,5-thiadiazoles having the formula: ##STR6## and processes for their preparation, wherein the variables A, m, Z and n are similar to the corresponding substituents of the compounds disclosed and claimed herein and wherein R.sup.1 may be hydrogen. However, the compounds disclosed therein are thiadiazoles, not oxadiazoles as described and claimed herein.